how to check if a number starts with two allowed numbers php? - php

Question

How to validate that the numeric string starts the first two numbers only with the following list allowed:
01, 02, 03, 04, 05, 06, 07, 08, 09, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, and 30.
$number = 1234567890;
if(ctype_digit($number)){
if (strlen($number) <= 10) {
echo "The registered number is correct";
} else {
echo "The number must have 10 digits";
}
} else {
echo "Number must be digit / numeric only.";
}
What I want to add to this functionality is to validate that said number that is stored in $number must start with the first two numbers that are in the list in order to continue.

Answer 1

Another way is to temporarily cast the number into a string and make use of the regular expression of ^([01]\d|2[0-4]|30) to check whether the number string starts with the specific values you mentioned in the above. You can use PHP's preg_match function to help check whether the string matches the regular expression.
So your code becomes:
if(ctype_digit($number)){
if (strlen($number) <= 10 and preg_match('/^([01]\d|2[0-4]|30)/', (string)$number)) {
echo "The registered number is correct";
} else {
echo "The number must have 10 digits or it begins with incorrect values";
}
} else {
echo "Number must be digit / numeric only.";
}
Regular expression explanation:
^: Matches the pattern from the start of the string
[01]\d: Matches 00, 01, 02, ... , 10, 11, ... , 19
2[0-4]: Matches 20, 21, ... , 24
30: Matches 30
([01]\d|2[0-4]|30): Match either [01]\d, 2[0-4] or 30
preg_match function returns 1 if the regular expression pattern is matched successfully.

Answer 2

If your allowed starting numbers are stored in an array, you could extract the 2 first numbers from $number using substr() and then check with in_array().

Answer 3

You can use regular expressions for this, but something to note: a number starting with a 0 will be interpreted as octal and converted to base 10. To avoid this, you must quote the number:
$number = '055231';
// Fill the list of allowed numbers
$allowed = array_map(function($number) {
// Fill each element to length 2 with '0' to the left
return str_pad($number, 2, '0', STR_PAD_LEFT);
}, range(1, 24));
$allowed[] = 30;
if (preg_match('/^\d{2,10}$/', $number)) {
// At this point we know it contains only numbers
// Extract first two characters
$start = substr($number, 0, 2);
// Check if it's in the allowed list
if (in_array($start, $allowed)) {
echo "The registered number is correct";
} else {
echo "The number is incorrect";
}
} else {
echo "Number must be digit / numeric only.";
}
Regex breakdown:
^ and $ are delimiters. The string must match exactly.
\d. Digit character class. Matches only numbers.
{2,10}. Previous group must be of length between 2 and 10 (since it must start with two digits).
Function reference:
range
array_map
str_pad
in_array
substr

Answer 4

A possible alternate solution could be to hardcode the valid prefixes and check if the first 2 characters of the string is one of them using array_key_exists:
$valid_prefixes = array_fill_keys(array("01", "02", "03", "04", "05",
"06", "07", "08", "09", "10",
"11", "12", "13", "14", "15",
"16", "17", "18", "19", "20",
"21", "22", "23", "24", "30"), true);
function starts_with_valid_prefix($str) {
global $valid_prefixes;
return strlen($str) >= 2 && array_key_exists(substr($str, 0, 2), $valid_prefixes);
}